This invention relates to a modified vegetable protein adhesive binder and a process for producing the same.
Vegetable protein materials are well known as adhesive binders for pigment containing coatings that are used for the coating of paper. The pigment containing coatings provide the paper with a desirable finish, gloss, and smoothness. The functions of the pigment in the coating are to fill in the irregularities of the paper surface and to produce an even and uniformly absorbent surface for printing. The adhesive functions to bind the pigment particles to each other as well as to the surface of the coating. The selection of a suitable adhesive is, therefore, an important factor in the quality of a coating for paper.
Vegetable protein materials have been extensively used as the adhesive binders for paper coatings and among these materials have been the vegetable protein isolates, typically soy isolate. Isolated soy protein is produced by the treatment of oil free soybean flakes with an alkaline solution to dissolve the protein which is removed from the non-soluble materials by filtration or centrifugation. The protein is then recovered from the solution by the addition of acid in order to precipitate the protein at its isoelectric point. The precipitated protein is then in an unhydrolyzed or generally unmodified state and can be dried and subsequently dispersed in an alkaline medium to form an adhesive binder for paper coating pigments.
The unhydrolyzed or unmodified soy protein isolates are less desirable as adhesives binders for paper coatings because of low solubility in weak alkaline solutions, high solution viscosity and sensitivity to heat, tending to form gels at temperatures of about 160.degree. F. Because of these disadvantages, unmodified or non-hydrolyzed soy protein is usually modified in some way to lower the viscosity of coating compositions containing soy protein and increases the adhesive strength of the protein when it is dispersed in weak alkaline solutions. Modification or hydrolysis of the protein also reduces the sensitivity of the protein to heat. Modification of the protein usually consists of hydrolysis or treatment of the extracted protein in an aqueous dispersion with various alkaline reagents under controlled conditions of pH, temperature and time. These conditions dissociate the native or unmodified protein structure into smaller subunits thereby improving the suitability of this material as a binder in paper coatings.
Another means of modifying the protein is to treat the unmodified protein material with various chemical reagents in order to alter the rheological properties of coatings containing the modified protein material as a binder. Generally, the purpose of modifying the protein is to provide for increased fluidity of the protein material in alkaline dispersions and also much lower viscosity of coating compositions prepared with the modified protein material as an adhesive binder.
U.S. Pat. No. 2,862,918 describes such a modified protein wherein an isolated soy protein which has been hydrolyzed beyond the initial gel stage is acylated by treatment of the hydrolyzed soy protein material with a carboxylic acid anhydride, such as acetic anhydride to acylate the protein. The acylated or modified protein when dispersed in alkali has a lower viscosity and is suitable as an adhesive binder in pigment containing coating composition for paper and the like. Hydrolysis is carried out prior to or commensurate with the reaction with the carboxylic acid anhydride but in any event is sufficient to insure that the initial gel stage of the protein material has been traversed. Typical times of hydrolysis noted in the patent are between 4 and 6 hours.
U.S. Pat. No. 2,932,589 describes a paper coating composition and the process for producing the same in which a modified soy protein material is obtained by reaction of an isolated soy protein with a dicarboxylic acid anhydride such as phthalic anhydride. This phthalated protein when "cut" without alkali disperses more readily than untreated protein and has a lower viscosity than untreated isolated protein. This reduction in viscosity of the alkali dispersion enables one to produce a coating composition which has a high solids level, with relatively good fluidity for adhesive application to the surface to be coated. It is also noted that increased adhesiveness is a result of the modification of the protein material with the phthalic anhydride.
In spite of the improvements obtained by modification of the protein material as described in the above two references, it is still desirable to obtain a modified protein material having improved rheological properties but produced by a process which is easier and more convenient to perform than those of the prior art. These objectives were achieved in the present invention by the present process which fulfills these objectives wherein a modified protein material suitable as an adhesive binder is obtained which additionally results in an unexpected improvement in rheological and paper coating properties.
It is therefore an object of the present invention to produce a modified proteinaceous adhesive binder of good rheological properties.
It is a further object to produce a modified vegetable protein adhesive binder with good pigment binding and paper coating characteristics.
It is a further object to modify the vegetable protein adhesive material by a process to produce an adhesive binder that is both reliable and convenient to practice.